The present invention relates to sensing and monitoring fluid pressure in a control system of the type having electrically operated control devices for performing various functions during the operation of an apparatus to be controlled.
In certain applications, as for example water level sensing in domestic appliances such as household washing machines, it is required to provide a sensor or transducer which is relatively inexpensive. Heretofore, such devices have comprised diaphragm actuated snap action switches for single level sensing of tub overfill, whereby actuation of the snap acting switch mechanism breaks a circuit to an electrically controlled water fill valve and/or energizes a drain valve or drain pump. The aforesaid type snap acting pressure switches for washing machines have typically been employed in washing machines having a timer actuated program cam for sequencing a series of electrical switches for controlling machine operations during the service or duty cycle of the washing machine.
However, it has been desired to provide control of a domestic washing machine in a manner employing solid state circuitry, and in particular a microprocessor, and thereby eliminate the need of the electromechanical programmer/timer. In utilizing such a microprocessor controller for a washing machine, it is necessary to provide a continuous electrical signal for monitoring the level of water in the washing machine tub. Thus the need has arisen for a low cost, relatively simple, water level sensor or transducer which has a high degree of sensitivity to changes in fluid pressure at low pressure levels as would be represented by the difference in the hydrostatic head of the water in a washing tub.
Fluid pressure sensors or transducers which provide a continuous electrical signal indicative of sensed fluid pressure are known in the art and various techniques have been employed. In particular, it is known to provide a diaphragm acting against a piezo electric crystal for providing a continuous electrical output proportional to the diaphragm force acting against the piezo electric device. Also, it is known to provide a fluid pressure responsive diaphragm acting against a slider block for moving a plurality of electrical wiper contacts along contact strips mounted on a printed circuit board, with the contact strips staggered for digitally coded output. In devices of this latter type, the length and spacing of the contact strips determine the increments of movement, and therefore pressure increment which must occur before the change in signal output is provided. The incremental pressure change sensitivity of such devices is thus necessarily quite limited.
Furthermore, piezo electric devices are complicated and costly to manufacture and generally it has been found difficult to provide such a device having high resolution or sensitivity for low pressure sensing and to have such a device lend itself to low cost manufacturing techniques.